Giant magnetoresistant single film alloys

ABSTRACT

A single layer film is deposited onto a substrate at room temperature from two sources, one source being a magnetic material, the other being a non-magnetic or weakly-magnetic material. The film is annealed for predetermined time in order to induce phase separation between the magnetic clusters and the non-magnetic matrix, and to form stable clusters of a size such that each magnetic particle, or cluster, comprises a single domain and has no dimensions greater than the mean free path within the particle.

This invention was made with Government support under Grant No.NSF-90-10908, awarded by the National Institute Science Foundation. TheGovernment has certain rights in this invention.

This is a continuation of application Ser. No. 08/255,077, filed Apr. 8,1994, abandoned, which is a continuation of application Ser. No.07/899,431, filed Jun. 16, 1992, abandoned.

BACKGROUND OF THE INVENTION

The drive towards higher density data storage on magnetic media hasimposed a significant demand on the size and sensitivity of magneticheads. This demand has been met, in part, by thin film inductive andmagnetoresistive heads which can be fabricated in very small sizes bydeposition and lithographic techniques similar to those used in thesemiconductor industry. Thin film inductive heads are subject to thesame problems as their core-and-winding predecessors of extremesensitivity to gap irregularities and stray fields which result inoutput signal losses. Thin film magnetoresistive heads, on the otherhand, rely on changes in the material's resistance in response to fluxfrom the recording media and do not require precise gap modeling. Forthese reasons, inter alia, magnetoresistive elements are increasinglypreferred over inductive heads for reading data stored at high densitieson magnetic media.

A figure of merit for magnetoresistive (MR) elements is ΔR/R, which isthe percent change in resistance of the element as the magnetizationchanges from parallel to perpendicular to the direction of the current.Current magnetoresistance elements are made from permalloy (81% Ni/19%Fe), which, at room temperature has a ΔR/R of about 3%. For improvedresponse, a higher value of ΔR/R is desirable.

Recently, it has been found that magnetic layered structures withanti-ferromagnetic couplings exhibit giant magnetoresistance (GMR) inwhich, in the presence of a magnetic field, ΔR/R can be as high as 50%.The GMR phenomenon is derived from the reorientation of the singledomain magnetic layers. For optimum properties, the thickness of themultilayers must be less than 3 nm, and ΔR/R increases with the numberof pairs of thin film layers. Thus, these multilayers providesignificant challenges for production because of the precision withwhich the thicknesses and other features, such as interface roughness,must be maintained for the many iterations of the pairs of magnetic andnon-magnetic films. Several studies have shown that GMR oscillates inmagnitude as a function of the thickness of the non-magnetic layers,increasing the concern about thickness control. These layered structuresare also subject to output noise from magnetic domains, and, since theiroutputs are nonlinear, the devices must be biased to obtain a linearoutput. Most reported work has been on Fe/Cr superlattices, however,Co/Cr, Co/Cu and Co/Ru superlattices have also been found to exhibitGMR.

The extreme sensitivity to layer thickness places significantlimitations on practical and economical application of GMR to datarecording and other potential uses. It would be desirable to provide amethod for forming GMR materials which is relatively insensitive tothickness and does not require multiple layers, and where the materialis not subject to output noise caused by domains or to thenonlinearities of the layered structures. It is to such a method andmaterial that the present invention is directed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide heterogeneous alloyswhich exhibit giant magnetoresistance.

It is a further object of the present invention to provide a method forforming such alloys.

In the preferred embodiment, a single layer film is sputtered onto asubstrate at room temperature from separate targets, one target being aferromagnetic material, the other being a non-ferromagnetic orweakly-magnetic material. The film is annealed for a predetermined timein order to induce phase separation between the magnetic clusters andthe non-magnetic matrix, and to form stable clusters of a size such thateach magnetic particle, or cluster, comprises a single domain and has nodimension greater than the mean free path within the particle.

Other deposition and film-forming techniques may be used includingsputtering from a single composite target, evaporation, metal pastes,mechanically combining the magnetic and non-magnetic materials orimplanting the magnetic materials (ions) into the non-magnetic matrix.

While a distinct interface needs to be maintained between the magneticand non-magnetic components of the film, the film can be formed frommaterials which are either immiscible or miscible under equilibriumconditions. In the latter case, deposition conditions can be controlledto assure that the desired interfaces are formed between the magneticand non-magnetic components of the film.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention will be facilitated byconsideration of the following detailed description of a preferredembodiment of the present invention, taken in conjunction with theaccompanying drawings, in which like reference numerals refer to likeparts and in which:

FIG. 1 is a plot of resistance ratio with applied magnetic fields forthe inventive film; and

FIG. 2 is a plot of resistance ratio with temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A single layer film comprising a magnetic material and a non-magnetic,or weakly-magnetic, material (hereinafter collectively identified as"non-magnetic") is deposited on a substrate by d.c. magnetronco-sputtering from separate targets. The sputter deposition is performedat low pressures, in the 10⁻⁷ torr range. The film is formed with thenon-magnetic film providing a matrix within which magnetic particles orclusters are precipitated. After deposition, the sample may be annealedto control the size of the particles. The ideal particles must be largeenough to avoid superparamagnetism (thermally-activated magnetizationreversal at room temperature), but small enough that their dimensions donot exceed the mean free path within the particles, and so that theyremain a single magnetic domain. In an actual sample there will be somevariation in particle size within a given film, with some particlessmaller than, and others larger than, the "ideal". The average particlesize in such a sample should possess the desired relationships to domainand mean free path.

A number of other deposition or film-forming techniques may also beused, including evaporation, pastes or mechanically-formed metals, e.g.,heated and compressed by high pressure rollers. Magnetic materials mayalso be implanted into a non-magnetic matrix. Any of these or similartechniques can then be followed by heat processing to assure formationof the desired magnetic precipitates.

By precipitating small magnetic particles in a non-magnetic matrix, anincreased surface area of magnetic material is made available for theelectron scattering that is responsible for magnetoresistance (MR). Thisdependence upon available surface area indicates that the magnetic andnon-magnetic materials must remain separate with distinguishableinterfaces. This requirement follows the explanations of MR inmultilayered structures that the electrons are scattered at theinterfaces, where spin dependent scattering predominates. The need fordistinct interfaces between the magnetic and non-magnetic materials doesnot, however, limit the choice of materials to those that are mutuallyinsoluble. While the materials can be immiscible under equilibriumconditions, they can also be miscible, with the materials being keptseparate by controlling deposition conditions.

In initial evaluations, cobalt-copper (Co-Cu) films were prepared byd.c. magnetron sputtering from separate copper and cobalt targets onto asilicon wafer having 100 orientation. A 3.5 minute pre-sputter step wasperformed prior to deposition. Background pressure was 6×10⁻⁷ torr. Thedeposition step of approximately 100 minutes at room temperature withthe substrates rotated above the targets at one revolution per secondprovided a 3,000 Å film.

Sputter rates were adjusted to yield films of 12, 19 and 28 atom percentcobalt. These samples exhibited GMR at 10°K with MR negligible at roomtemperature, indicating a superparamagnetic behavior due to a highlydisordered state and fine grain size. The samples were annealed toincrease grain size, to achieve phase separation between cobalt andcopper, and to form stable cobalt particles. After annealing, the 19 Coand the 28 Co samples show the largest MR changes. Their MR curves hadthe shape shown in line C of FIG. 1. The maximum MR occurred at thecoercive force, H_(c), which was approximately 500 Oe at 10°K for allannealed 19 Co and 28 Co samples. Remanence/saturation (M_(r) /M_(s))ratios were greater than 0.3 at 10°K for all annealed samples. BothH_(c) and M_(r) /M_(s) decreased with increased temperature atmeasurement and annealing time. The magnetic behavior of the annealedsamples was associated with the precipitation of cobalt-rich particlesin a copper-rich matrix.

FIG. 2 shows ΔR/R versus temperature for as-deposited and annealed 19 Coand 28 Co specimens. Saturation fields for the MR coincided with thesaturation fields for magnetization. The MR ratio increased withdecreasing annealing temperature and time (except for the as-deposited28 Co).

As annealing times and temperatures increase, the average Co-richparticle sizes also increase, with corresponding decrease in MR. LargerCo particles have several adverse effects on MR: 1) the surface/volumeratio decreases, reducing the spin-dependent interfacial scatteringrelative to bulk-scattering processes; 2) the particles become largerthan the mean-free path within the particles; and 3) the particles areno longer single domains such that the interaction of the conductionelectron spins with the varying magnetization distribution in theparticles produces a state in which the conduction electron spinchannels are mixed. Also seen in FIG. 2 is the rapid relaxation rate ofMR with increasing temperature, which is attributable tosuperparamagnetism.

The GMR in the heterogeneous copper-cobalt alloys may be analyzed in thesame manner as the copper-cobalt multilayers. Assuming a randomdistribution of cobalt particles with average radius r_(Co) in a coppermatrix, and adopting a spin-dependent scattering model at the surface ofcobalt particles and within the cobalt particles, the conductivity canbe written as: ##EQU1## where n is the number of electrons; e is theelectron charge, m is the electron mass, and Δ.sup.σ is the averagescattering matrix. The phenomenological input for Δ.sup.σ is: ##EQU2##and c is the Co concentration; λ_(Cu) and λ_(Co) are the mean free pathsof Cu and Co, respectively; ξ is the scattering strength for surfaces;p_(Co) and p_(s) are the spin dependent ratios for scattering within theCo particles and at their surfaces, respectively. Thus Equation (1) isthe sum of scattering in Cu, Co, and at the interfaces between them.Since ##EQU3## Equation (2) is substituted into Equation (1), andEquation (3) becomes: ##EQU4## with ± referring to spin up and down, and##EQU5##

In Co/Cu multilayers, the principal spin dependent scattering is fromthe interfacial term (p_(s) =0.5 p_(Co) =0.2, ξ=0.3) 16!. Thus, ifp_(Co) =0, Equation (4) reduces to ##EQU6## Equation (5) correctlypredicts the inverse dependence of MR on the particle size, inaccordance with the surface/volume ratio consideration noted above.

A consideration in the development of magnetoresistive films forpractical applications is that the applied saturation field be as low aspossible while still achieving the maximum ΔR/R. It is well known thatsoft ferromagnetic materials provide greater MR with lower appliedfields. Materials which may be used as softer magnetic particles includethose which are well known in the recording industry for their use ininductive heads, including iron, cobalt-iron, and permalloy.

Another factor which will influence the efficiency of the saturationfield in inducing magnetoresistance is the shape of the magneticparticles. A demagnetizing field will be generated by a sphericalparticle such that an additional field must be overcome by the appliedsaturation field. By controlling the shape of the particles duringdeposition, disc-like particles can be formed which possess lowerdemagnetization fields while still having large surface areas.Preferably, the plane of the disc-like particles will be orientedparallel to the field. Such an effect can be achieved by control ofdeposition parameters or by post-deposition anneal under a magneticfield.

For practical applications, a robust material such as silver may bedesirable for use as the non-magnetic matrix. Cobalt and silver areimmiscible under equilibrium conditions. After annealing one hour at200° C., the ΔR/R at room temperature for a sample of 33 atom-% Co insilver was measured at 21.5%. An advantage of using silver is itsrelatively high environmental stability, i.e., minimal corrosion oroxidation, and such an alloy system is much easier to prepare andcontrol than multilayers. Silver is further suited for use in such anapplication because none of the magnetic elements are soluble in silver.Other possible matrix materials include ruthenium, gold and chromium,among others. It is also desirable to supplement or substitute thecobalt, which is a hard magnetic material, with softer magneticmaterials.

The above-described method eliminates the need for use of multilayersfor achieving giant magnetoresistance. The single layer film of thepresent invention possesses several advantages over the prior GMRmaterials in that it is easier to control fabrication, its output may belinear, and there are no domains so that there are no domain walls toproduce noise. It is anticipated that the inventive film willsignificantly enhance the fabrication of MR heads, making such filmsmore practical and economical than those of the current technology.

It will be evident that there are additional embodiments which are notillustrated above but which are clearly within the scope and spirit ofthe present invention. The above description and drawings are thereforeintended to be exemplary only and the scope of the invention is to belimited solely by the appended claims.

I claim:
 1. A method for forming a giant magnetoresistant film, themethod which comprises:co-depositing a magnetic material and asubstantially non-magnetic metallic material on a substrate to create asubstantially homogeneous film comprising a plurality of magneticparticles in a substantially non-magnetic metallic matrix; and annealingsaid film for a length of time determined by a desired size of saidmagnetic particles, said magnetic particles becoming larger withincreased anneal time and said desired size being less than a mean freepath within said magnetic material and such that an amount ofspin-dependent interfacial scattering from an outer surface of saidmagnetic particles is increased relative to an amount of bulk scatteringwithin said magnetic particles to increase giant magnetoresistance inaccordance with the relationship ##EQU7## and r_(MR) is an averageradius of said magnetic particles, p_(s) is a spin-dependent ratio forscattering at a surface of said magnetic particles, λ_(MX) is a meanfree path in said substantially non-magnetic matrix, λ_(MR) is a meanfree path in said magnetic particles, ξ is a scattering strength forsaid surface of said magnetic particles, and c is a concentration ofsaid magnetic particles.
 2. A method as in claim 1 wherein said desiredsize of said magnetic particles is selected so that said magneticparticles are single domains.
 3. A method as in claim 1 wherein the stepof forming a composite includes forming said magnetic particles in agenerally-flattened shape.
 4. A method as in claim 1 wherein saidmagnetic material is cobalt.
 5. A method as in claim 1 wherein saidmagnetic material is permalloy.
 6. A method as in claim 1 wherein saidmagnetic material is iron.
 7. A method as in claim 1 wherein saidmagnetic material is cobalt-iron.
 8. A method as in claim 1 wherein saidsubstantially non-magnetic material is copper.
 9. A method as in claim 1wherein said substantially non-magnetic material is silver.